Add instruction scheduling model using SSA model

Add instruction scheduling model using SSA model

As promised, here is a new model :-) So far the results are very good.

Basically, I model the dependencies between the instructions using a SSA
assignment. It behaves a bit like a Petri network, passing tokens
between the "instructions". When instructions have multiple dependencies
as input, they wait for all their input tokens to be ready before they
execute. It assumes we have an infinite number of registers available to
hold the variables, given this is an "abstract" architecture.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>